In the plastic molding industry it is a common practice to use virgin pellets of polymer resin to make a product and to uniformly blend a color additive with the pellets before molding and in just the right proportion to obtain the desired color of the product. Accurate control of the color is imperative so that similar or different molded products molded at different times will match if used together. For this purpose the amount of color additive is specified as a percentage of the virgin material. Whenever a molded part is imperfect, it is ground up and the resulting "regrind" material is mixed with virgin material to make another product of the same color as the regrind material. In this case the percentage of color additive is still based on the amount of virgin pellets and not on the total batch size.
Two standard ways of supplying components to a blender or mixer in the desired proportion are by weight ratio and by volume ratio. In either case the amounts of virgin material and regrind are determined and the required amount of color additive is calculated. In the case of weight ratio mixing, each of the three components is weighed out and then mixed together. This is done, for example, by vacuum conveying each component to a hopper, constantly weighing the hopper, and shutting off the material supply when the correct amount of the respective material has been received. Color or other additives which comprise a relatively small portion of the whole batch may be weighed separately and then added to the batch. With this system some overrun of each of the materials is possible as a consequence of which the resulting ratio of the virgin material to color may differ from the specified ratio.
Volumetric supply of materials is usually carried out by auger conveyors which carry each component to the mixer. A pickup coupled to each auger motor generates a certain number of counts per auger revolution. By calibrating an auger for use with a given material the number of counts per unit volume of material is determined. The desired amount of each component is predetermined, the amounts are expressed in terms of counts, and the auger for each component is run for the respective number of counts. This should give the correct amount of each component, provided the density of each remains consistent after calibration and that any overrun which occurs is also consistent. However the opportunity for errors is present.